1. Field of the Invention
This invention relates to developing core loading pattern designs for a core of a nuclear reactor.
2. Related Art
A nuclear reactor such as a boiling water reactor (BWR) or pressurized water reactor (PWR), for example, may operate from about one to two years on a single core loading of fuel. Upon completion of a given period (energy cycle), approximately ¼ to ½ of the least reactive fuel (oldest or most burnt) may be discharged from the reactor.
The operation of the cycle may depend on the placement of the fuel assemblies (fresh fuel, once-burnt fuel, twice-burnt fuel, etc.). Due to the presence of burnable poisons in the core, such as gadolinium, for example, the characteristics of the fresh fuel, once-burnt fuel, and twice-burnt fuel assemblies may be different. The fresh fuel assembly is typically less reactive at the Beginning-of-Cycle (BOC), as compared to a once-burnt fuel bundle, due to the presence of gadolinium. At the End-of-Cycle (EOC), since most or all of the poison has burnt out, the fresh assemblies are typically more reactive than the once-burnt fuel. Although the shape of an exposure dependent reactivity curve of the twice-burnt fuel may be similar to that of the once-burnt fuel, the reactivity of the twice-burnt fuel is smaller in magnitude. By combining fresh, once-burnt, and twice-burnt fuel assemblies, however, a substantially even reactivity may be achieved across the core, throughout the energy cycle.
In addition to reactivity considerations, the placement of fuel assemblies (“fuel bundles”) may impact thermal limits, power shaping, and fuel cycle economics. If fuel bundles, too high in reactivity, are placed face-adjacent, inadequate margin to reactivity thresholds or thermal limits may result. Cycle length may also be increased by the placement of a greater number of reactive bundles toward the center of the core, rather than placing these reactive fuel bundles at the periphery of the core. Accordingly, a core loading pattern may define many of the most important considerations for a nuclear fuel cycle.
Traditionally, core loading design determinations have been made on a trial and error basis. A current process being used is a stand-alone manual core loading pattern design process that requires a designer to repeatedly enter reactor plant specific operational parameters into an ASCII text file, which is an input file. Data entered into the input file may include blade notch positions of control blades (if the evaluated reactor is a boiling water reactor (BWR)), core flow, core exposure, which may be the amount of burn in a core energy cycle, measured in mega-watt (or giga-watt days per short time (MWD/st, GWD/st), etc.
A Nuclear Regulatory Commission (NRC) licensed core simulation program reads the resulting input file and outputs the results of the simulation to a text or binary file. A designer then may evaluate the simulation output to determine if the design criteria has been met, and to verify that no violations of margins to thermal limits have occurred. Failure to meet design criteria (i.e., violations of one or more limits) require a manual modification to the input file. Specifically, the designer would manually change one or more operation parameters, and re-perform the core simulation program. This process was repeated until a satisfactory core loading pattern design was achieved.
This process is extremely time consuming. The required ASCII text files are laborious to construct, and often are error prone. The files are usually ASCII format and extremely long, sometimes exceeding one thousand or more lines of code. A single error in the file could result in a crash of the simulator, or worse, may result in a mildly errant result that could be hard to initially detect, but which would profligate with time and iterations to perhaps reduce core cycle energy, if an actual operating nuclear reactor core was loaded in accordance with the erroneous core loading pattern.
Further, no assistance is provided via the manual iterative process in order to guide a designer toward a more favorable core loading pattern design solution. In the current process, the responsible designer or engineer's experience and intuition are the sole means of determining a core loading pattern design solution.